The present invention relates to a seat occupancy sensor, and in particular, a sensor for detecting seat occupancy in a vehicle.
One type of a seat occupancy sensor is known from German Patent Application 10 2004 015 408.2 that is usable especially in non-safety-critical applications, e.g., a seat belt warning system, to warn a passenger sitting on the vehicle seat to fasten his/her seat belt. Such a warning system outputs an acoustic or optical signal, for example, when the vehicle seat is occupied but the belt lock of the respective seat belt has not yet been fastened. The seat belt sensor in such a system essentially differentiates between seat occupancy by a passenger and seat occupancy by an object, such as a handbag. A more complex classification of seat occupancy is not used.
An object of the present invention is to provide a functional diagnosis in connection with a seat occupancy sensor.
This object is achieved according to the present invention by a seat occupancy sensor having two pressure-activatable switch elements assigned to two areas of a seat spaced a distance apart from one another and connected in a logic AND operation arrangement. An electric resistance element is connected in parallel to each switch element, with the resistance value of each resistance element being between the resistance value with an activated switch element and the resistance value with an unactivated switch element. The first and second switch elements may be connected in series, and the first and/or second switch element may include a plurality of individual sensor cells interconnected in a logic OR operation arrangement.
In the simplest case, the electric resistance of the switch elements is zero for an activated switch (switch closed) and is infinite for a deactivated switch (switch open). Without the embodiment according to the present invention, it would be impossible to differentiate between a line interruption and the deactivated state. If the logic AND operation is implemented by a series connection of the two switch elements, this yields unique values for the total resistance of the seat occupancy sensor. These values are usually clearly differentiable from one another in the event of a fault and in the event of proper functioning in the four possible operating states, namely: only one or the other switch is activated, both are deactivated, or both are activated.
In one embodiment, the first and second switch elements are connected in series with the electric resistance element connected in parallel to each switch element.
It should be pointed out that the seat occupancy sensor may have more than two switch elements. For example, three or more switch elements may be wired in a logic AND operation. Alternatively, several groups of two or more switch elements may be provided in an AND operation.
In a preferred embodiment of the invention, the first and/or second switch elements include a plurality of individual switch cells that are interconnected in such a way as to implement a logic OR operation. The individual switch cells of the switch elements may be wired in parallel, for example. The wiring of the individual switch cells in an OR operation causes the switch elements to be triggered when a single switch cell is triggered by a pressure force. If the individual switch cells are arranged so they are distributed in a cluster over a certain zone of the vehicle seat, then this zone constitutes the active area of the respective switch elements. Occupancy at any location within this zone is thus recognized as occupancy by the switch element.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.